This invention relates to a device comprising:
a load which can be powered by a battery, and PA1 a power-supply circuit for charging the battery and/or powering the load. PA1 first detection means for detecting expiration of a discharge time of the battery, PA1 second detection means for detecting a battery voltage substantially equal to a threshold value, PA1 a discharger for discharging the battery, with the load switched off, after detection of expiration of the discharge time until detection of the threshold value, PA1 first time-measurement means for measuring the time during which the load is powered by the battery after detection of expiration of said discharge time until the threshold value is reached, PA1 second time-measurement means for measuring the time during which the discharger has been operative, and PA1 correction means for correcting the expired discharge time depending on the magnitude of the weighted sum of the time intervals measured by the first and the second time-measurement means relative to a reference value. PA1 third detection means for detecting a weighted sum of the time intervals measured by the first and the second time-measurement means, which sum is substantially equal to the reference value, and for switching off the discharger or the load after detection of said sum.
In the present application "battery" is to be understood to mean a unit comprising one or more cells, such as nickel-cadmium cells.
Such a device is constituted by, for example, a domestic appliance which can be battery-powered or which can be mains-powered by means of a power-supply circuit. The battery can be recharged by the power-supply circuit when the appliance is switched off. In particular, such a device may be constituted by a rechargeable electric shaver.
Such a device is known from European Patent Application No. 110,466, which corresponds to U.S. Pat. No. 4,536,757 (8/20/85). In said known device a voltage is detected to generate a signal which informs the user that the battery is almost drained and must be recharged before long. This voltage is selected in such a way that below this voltage at least one period of use of, for example, approximately 5 minutes is available. Instead of a voltage it is also possible, as in a device in accordance with the present application, to generate a nearly-discharged signal after detection that a certain discharge time has expired. However, it is found that many users recharge the battery immediately after the occurrence of such a nearly-discharged signal. As a result of this, the batteries are never discharged completely. However, in the course of time the unused battery material becomes inert causing the battery capacity to decrease. As a result of this, the time still available for use, after the nearly-discharged indication, decreases, so that ultimately a full period of use is not available any more. Moreover, the battery capacity also decreases as a result of aging, so that the time still available for use after the nearly-discharged indication also decreases. In addition, in the known device the time still available for use depends on the extent to which the battery is loaded by the device. A related battery powered device is described in copending U.S. application Ser. No. 121,446 filed Nov. 16, 1987.